This invention relates to shadow mask type cathode ray tubes and, particularly to contours of shadow masks within such tubes.
In a shadow mask tube, a plurality of convergent electron beams are projected through a multiapertured color selection shadow mask to a mosaic screen. The beam paths are such that each beam impinges upon and excites only one kind of color-emitting phosphor on the screen. Generally, the shadow mask is attached to a rigid frame, which in turn, is suspended within the picture tube envelope.
Presently, all commercial color picture tubes have a front or viewing faceplate portion that is either spherical or cylindrical. However, it is desirable to develop a tube having a generally flat faceplate. According to prior art tube design concepts, in tubes having curved faceplates, the shadow mask is similarly curved so that it somewhat parallels the faceplate contour. Thus, in keeping with these prior art concepts, in a tube with a flat faceplate, the corresponding shadow mask should also have an almost flat contour. However, such a mask has insufficient self-supporting strength or rigidity. One way to provide this strength or rigidity would be to put the mask under tension as is done in some commercially available tubes having cylindrical faceplates. However, tension methods require undesirable and expensive frame structures. Another recently suggested way of providing strength to the mask is to corrugate it.
In the manufacture of more conventional tubes having spherical faceplates, another recent devleopment is to provide a shadow mask with greater curvature than that of the faceplate. Because of the increase in curvature, the thickness of the mask-frame assembly in the direction of the tube's longitudinal axis is increased thereby causing the frame to extend beyond the faceplate sidewalls. Such extension is undesirable since the mask-frame assembly is exposed to possible damage during manufacture. Therefore, it is desirable to develop a mask-frame assembly of reduced thickness. Suggested methods of accomplishing this reduction include telescoping the mask within the frame or distorting the mask skirt.
Embodiments of the present invention provide another way of obtaining a mask having high rigidity for use with a flat faceplate while other embodiments permit reduction in mask-frame assembly thickness for tubes with spherical faceplates.